How is a $12 billion valuation dropping the ball. Pinterest appeals to a very lucrative democratic who are looking for stuff to buy, such as clothes, household products, and other stuff, whereas people go to Facebook to see pictures of family or engage in political gossip.
For fashion, afaik "influencers" are indeed using platforms like Instagram much more than Pinterest. That is certainly a situation where they could have defended their turf more effectively.
If I see something on Pinterest I'm interested in, then I'll go to Amazon to see if it's offered there, and if not, I'll head over to Google to see if I can get it cheaper. I'm not going to buy directly from Pinterest unless it's a link directly to Amazon.